Method for producing 2-methylbutyric acid having a reduced content of 3-methylbutyric acid from the secondary flows arising in the production of pentanoic acids

ABSTRACT

A process for preparing 2-methylbutyric acid having a reduced content of 3-methylbutyric acid from the secondary streams obtained in the preparation of pentanoic acids, includes generating a stream enriched with 2-methylbutanal and 3-methyl-butanal. The stream enriched with 2-methylbutanal and 3-methylbutanal of step is reacted with formaldehyde. The reaction with formaldehyde is followed by removal of the organic phase and selective hydrogenation in the presence of a hydrogenation catalyst with hydrogen at elevated temperature and elevated pressure and, after removal of the hydrogenation catalyst, treatment of the hydrogenation output obtained with an oxidizing agent.

CLAIM FOR PRIORITY

This application is a national phase application of PCT/EP2014/003056FILED Nov. 14, 2014 which was based on application DE 10 2013 020 320.1FILED Dec. 5, 2013. The priorities of PCT/EP2014/003056 and DE 10 2013020 320.1 are hereby claimed and their disclosures incorporated hereinby reference.

TECHNICAL FIELD

The present invention relates to a process for preparing 2-methylbutyricacid having a reduced content of 3-methylbutyric acid from secondarystreams obtained in the preparation of pentanoic acids.

BACKGROUND

Pentanoic acids, also called valeric acids, have gained economicsignificance as intermediates in industrial organic chemistry. Theyoccur in four different structural isomers as linear n-pentanoic acids,branched 2-methylbutyric acid, branched 3-methylbutyric acid and highlybranched pivalic acid. Pentanoic acids can be used as such, for examplefor the production of fragrances. In addition, esters of pentanoic acidswith polyols are finding increasing significance as lubricants. Thepentanoic acids can be used in pure isomeric form or in the form of anisomer mixture (Weissermel, Arpe, Industrielle Organische Chemie[Industrial Organic Chemistry], 3rd edition, VCH VerlagsgesellschaftmbH, Weinheim, 1988, page 218; Schneidmeir, Chemiker-Zeitung, volume 96(1972), no. 7, pages 383-387). The isomeric pentanoic acids are producedindustrially by oxidation of the corresponding pentanals, or in the caseof pivalic acid by hydrocarboxylation (Ullmann's Encyclopedia ofIndustrial Chemistry, 6th edition, 2003, Wiley-VCH Verlag GmbH & Co.KGaA, Weinheim, vol. 6, pages 495-503).

The pentanal starting compounds are prepared industrially by reaction ofbutenes with synthesis gas, a mixture of carbon monoxide and hydrogen,in the presence of transition metal compounds. The reaction of olefinswith synthesis gas is also referred to as the hydroformylation reactionor oxo reaction, and the hydroformylation of but-1-ene affords, as wellas the straight-chain n-aldehyde n-pentanal, also certain proportions ofthe isoaldehyde 2-methylbutanal (Ullmann's Encyclopedia of IndustrialChemistry, 6th edition, 2003, Wiley-VCH Verlag GmbH & Co. KGaA,Weinheim, vol. 2, pages 73-74; vol. 25, pages 286-289).

Butenes are obtained industrially by the steamcracking of naphtha.Typically, separation of 1,3-butadiene from the butene cut from thenaphtha cracking to form raffinate I is followed by separation ofisobutene to form raffinate II (Weissermel, Arpe, IndustrielleOrganische Chemie, 3rd edition, VCH Verlagsgesellschaft mbH, Weinheim,1988, pages 71-79). For the subsequent hydroformylation reaction,predominantly raffinate II is used, in which a small residual isobutenecontent can be permitted. In special cases, it is also possible toprocess raffinate I having a high isobutene content, and occasionallyalso a but-1-ene-depleted raffinate II, which can also be referred to asraffinate III.

The hydroformylation reaction can be conducted either in the presence orin the absence of complex-forming compounds, for example in the presenceof organophosphorus compounds. According to EP 0 366 089 A2, ahomogeneous organic solution is employed with rhodium triphenylphosphinecatalysis. Since a maximum proportion of n-pentanal compared to2-methylbutanal in the pentanal mixture formed is generally the aim, thehydroformylation reaction is frequently conducted in the presence ofhomogeneously dissolved transition metal complexes, which first enableisomerization of the but-2-ene to but-1-ene, which is thenhydroformylated predominantly to n-pentanal. Rhodium complex catalystssuitable for the isomerizing hydroformylation of a mixture of linearbutenes are described, for example, in DE 102 25 282 A1, in which thecomplex ligands have a xanthene skeleton.

Rhodium complex catalysts based on bisphosphite ligands together withsterically hindered secondary amines, which are likewise suitable forthe isomerizing hydroformylation of a mixture of linear butenes, arediscussed in DE 10 2008 002 187 A1. Two-stage process variants are alsoknown, for example according to DE 43 33 324 A1, DE 42 10 026 A1, DE 10108 474 A1 and DE 101 08 475. In the first stage, preferably but-1-ene isconverted, while, in the second stage, the but-2-ene-containing offgasfrom the first stage is hydroformylated to give a mixture of n-pentanaland 2-methylbutanal. According to DE 43 33 324 A1 and DE 101 08 474 A1,the first hydroformylation stage can also be conducted in the presenceof water-soluble rhodium complex catalysts. In this type of reactionregime, a liquid aqueous catalyst solution is present alongside theliquid organic reaction solution, which, after leaving thehydroformylation zone, can be separated from one another in a simplemanner by phase separation. Because of the presence of an aqueous phaseand an organic liquid phase, this type of reaction regime is alsoreferred to as a heterogeneous process or biphasic process.

According to the composition of the butene feed mixture and the reactionconditions in the hydroformylation stage, a pentanal mixture is obtainedwith varying proportions of n-pentanal, 2-methylbutanal, 3-methylbutanaland a small amount of pivalaldehyde, which is typically distilled.Because of the small difference in boiling point between 2-methylbutanal(92° C. at standard pressure) and 3-methylbutanal (92.5° C. at standardpressure), 3-methylbutanal cannot be separated completely from the2-methylbutanal with acceptable distillation complexity and the2-methylbutanal obtained is used for the preparation of 2-methylbutyricacid with a residual 3-methylbutanal content of typically greater than0.2% by weight, based on the organic component. For particularapplications, for example for fragrance production or for specificlubricants, however, there is a demand for a 2-methylbutyric acidquality where the residual 3-methylbutyric acid content has to be below0.2% by weight, based on the organic component. A very low residualcontent of 3-methylbutyric acid below 0.2% by weight may also beimportant for n-pentanoic acid/2-methylbutyric acid mixtures. There istherefore a need for a process for preparing 2-methylbutyric acid havinga reduced residual content of 3-methylbutyric acid from the secondarystreams obtained in the preparation of pentanoic acids.

SUMMARY OF INVENTION

The present invention therefore consists in a process for preparing2-methylbutyric acid having a reduced content of 3-methylbutyric acidfrom the secondary streams obtained in the preparation of pentanoicacids. It is characterized in that

-   -   a) a mixture comprising linear butenes is reacted in the        presence of transition metal compounds of group VIII of the        Periodic Table of the Elements with carbon monoxide and hydrogen        at elevated temperature and elevated pressure to give a pentanal        mixture;    -   b) the mixture obtained in step a) is separated into a stream        enriched with 2-methylbutanal and 3-methylbutanal, and a stream        enriched with n-pentanal;    -   c) the stream enriched with 2-methylbutanal and 3-methylbutanal        of step b) is reacted with formaldehyde;    -   d) the reaction product obtained after step c) is treated with        an oxidizing agent; and    -   e) 2-methylbutyric acid having a residual 3-methylbutyric acid        content of less than 0.2% by weight, based on the organic        component, is removed by distillation from the reaction product        obtained in step d),        with the proviso that the reaction with formaldehyde in step c)        is followed by removal of the organic phase and selective        hydrogenation in the presence of a hydrogenation catalyst with        hydrogen at elevated temperature and elevated pressure and,        after removal of the hydrogenation catalyst, treatment of the        hydrogenation output obtained with an oxidizing agent in step        d).

DETAILED DESCRIPTION

Feedstocks for the process of the invention are hydrocarbon mixturestypically containing very small amounts, if any, of polyunsaturatedcompounds and acetylene compounds and containing at least one of theolefins cis-but-2-ene, trans-but-2-ene and but-1-ene.

In addition, the feed mixture may include varying proportions ofisobutene. Feed mixtures of this kind are industrially available asraffinate I, raffinate II or raffinate III.

The butene hydroformylation can be conducted in a homogeneous variant inthe organic reaction medium with dissolved transition metal catalysts ofgroup VIII of the Periodic Table of the Elements in the unmodifiedvariant, or in the variant modified with complex ligands. Particularlyeffective solvents in the organic reaction medium have been found to bethe higher-boiling condensation compounds of the pentanals, especiallythe trimers, which are obtained as by-products in the hydroformylation,and mixtures thereof with the pentanals to be prepared, and so a furtheraddition to the solution is not absolutely necessary. In some cases,however, an additional solvent may be found to be appropriate. Thesolvents used are organic compounds in which starting material, reactionproduct and catalyst are soluble. Examples of such compounds arearomatic hydrocarbons such as toluene and benzene or the isomericxylenes and mesitylene. Other commonly used solvents are paraffin oil,cyclohexane, n-hexane, n-heptane or n-octane, ethers such astetrahydrofuran, ketones or Texanol® from Eastman. When the homogeneousvariant in their presence is employed, suitable complex ligands aretriarylphosphines such as triphenylphosphine (EP 0 366 089 A2),diphosphines, for example those based on the xanthene skeleton (DE 10225 282 A1), phosphites as described, for example, in U.S. Pat. No.4,599,206, or diphosphites, for example described in EP 0 213 639 A2 andDE 10 2008 002 187 A1. It is also possible to use mixtures of complexligands, for example of triarylphosphines with phosphites ordiphosphites, as known from WO 2010/117391 A1, in the hydroformylationreaction.

Through the choice of the composition of the butene feed mixture and thehydroformylation conditions, it is possible to control the ratio ofn-pentanal to 2-methylbutanal.

If the aim is a maximum proportion of n-pentanal compared to2-methylbutanal in the hydroformylation mixture, it is advisable, aswell as a but-1-ene-rich feed stream, to use modified transition metalcatalysts which at first bring about isomerization of the residualbut-2-ene content to but-1-ene, which is then hydroformylatedpredominantly to n-pentanal. In one configuration of the process of theinvention, it is possible to use the hydroformylation catalysts that areknown from DE 102 25 282 A1 and have complex ligands based on thexanthene skeleton, or the hydroformylation catalysts that are known fromEP 0 213 639 A2 or DE 10 2008 002 187 A1 and are based on stericallyhindered diphosphites.

High proportions of n-pentanal can also be obtained in a two-stageprocess variant which is known per se from DE 101 08 474 A1 and DE 10108 475. In the first stage, which can likewise be conducted by theheterogeneous variant in the presence of water with water-solublecomplex ligands, for example with sulfonated phosphines such astriphenylphosphine with different degrees of sulfonation, predominantlybut-1-ene reacts in high selectivity to give n-pentanal, and thebut-2-ene-enriched offgas is subsequently converted in a second stageunder isomerizing conditions to a pentanal mixture having a highn-pentanal content. By combining the streams from the first and secondhydroformylation stages, it is possible to prepare a pentanal mixturehaving a high proportion of n-pentanal relative to 2-methylbutanal.

If the aim is a high proportion of 2-methylbutanal in the pentanalmixture, for example because of market circumstances, or said highproportion arises because of the composition of the butene feed mixture,preference is given to working in the absence of complex ligands in theunmodified mode of operation. In this case, the active hydroformylationcatalyst forms from the transition metal or transition metal compoundand carbon monoxide. It is assumed in the specialist literature that thetransition metal compound HM(CO)₄ is the catalytically active transitionmetal species in the unmodified transition metal catalysis.

With increasing isobutene content in the butene feed mixture, theproportion of 3-methylbutanal in the hydroformylation product alsoincreases.

In the modified variant, the molar ratio of transition metal to complexligands is generally 1:1 to 1:1000, but it may also be higher.Preference is given to using the transition metal and the complex ligandin a molar ratio of 1:3 to 1:500, preferably 1:50 to 1:300. The modifiedhydroformylation reaction of the butene feed mixture is typicallyconducted at temperatures of 50 to 160° C. and pressures of 0.2 to 15MPa. The transition metal concentration is generally 10 to 700 ppm,preferably 25 to 500 ppm, based on the reaction mixture.

If the unmodified variant is employed, the transition metal is used insmaller amounts, generally in an amount of 1 to 100 ppm, preferably 2 to30 ppm, based on the amount of butene used. It is appropriate to work athigher pressures in the range from 5 to 70 MPa, preferably from 5 to 60MPa and especially from 10 to 30 MPa. Suitable reaction temperaturesvary within the range from 50 to 180° C., preferably from 50 to 150° C.and especially from 100 to 150° C.

The composition of the synthesis gas can be varied within wide limits.In general, mixtures in which the molar ratio of carbon monoxide andhydrogen is 5:1 to 1:5 are used. Typically, this ratio is 1:1 ordeviates only slightly from this value in favor of hydrogen. The mixturecomprising linear butenes can be supplied to the reaction zone as suchor in solution with organic solvents, such as hydrocarbons.

The transition metals of group VIII of the Periodic Table of theElements used are preferably cobalt, rhodium, iridium, nickel,palladium, platinum, iron or ruthenium, and especially rhodium andcobalt. The modified or unmodified transition metal catalyst forms underthe conditions of the hydroformylation reaction from the transitionmetal compounds used, such as the salts thereof, such as chlorides,nitrates, sulfates, acetates, pentanoates or 2-ethylhexanoates, thechalcogenides thereof, such as oxides or sulfides, the carbonylcompounds thereof, such as M₂(CO)₈, M₄(CO)₁₂, M₆(CO)₁₆, M₂(CO)₉,M₃(CO)₁₂, the organo-transition metal compounds thereof, such ascarbonyl acetylacetonates or cyclooctadienyl acetates or chlorides. Itis possible here to use the transition metal compound in solid form orappropriately in solution. Particularly suitable transition metalcompounds which are used as catalyst precursor are rhodium pentanoate,rhodium acetate, rhodium 2-ethylhexanoate or cobalt pentanoate, cobaltacetate or cobalt 2-ethylhexanoate or Co₂(CO)₈, Co₄(CO)₁₂, Rh₂(CO)₈,Rh₄(CO)₁₂ or Rh₆(CO)₁₆ or cyclopenta-dienylrhodium compounds, rhodiumacetylacetonate or rhodium dicarbonyl acetylacetonate. Preference isgiven to using rhodium oxide and especially rhodium acetate, rhodium2-ethylhexanoate and rhodium pentanoate.

Alternatively, it is also possible first to preform the transition metalcatalyst in a pre-carbonylation stage and then to supply it to theactual hydroformylation stage. The preforming conditions generallycorrespond to the hydroformylation conditions.

The hydroformylation stage can be conducted either batchwise orcontinuously. The pentanal mixture formed is separated from thehydroformylation catalyst by conventional methods, for example bydistillation in the homogeneous process regime or by simple phaseseparation from the aqueous catalyst solution in the heterogeneous orbiphasic process regime.

The transition metal catalyst, optionally after addition of freshtransition metal compound and optionally fresh ligand if the modifiedmode of operation is being employed, and after removal of a portion ofthe aldehyde condensation products formed in the course of the reaction,is recycled into the reaction zone.

The requirement for a particular composition of the pentanal mixtureobtained with regard to the n-pentanal, 2-methylbutanal and3-methylbutanal isomers is guided by the market circumstances and can becontrolled via the composition of the butene feed mixture and via thechoice of the hydroformylation conditions. Frequently, the aim is apentanal mixture containing generally at least 85 mol % of n-pentanal,less than 15 mol % of 2-methylbutanal and, depending on the isobutenecontent, less than 5 mol % of 3-methylbutanal, preferably less than 1mol % and especially less than 0.2 mol % of 3-methylbutanal, based ineach case on the sum total of pentanals. But pentanal mixtures having ahigher proportion of 2-methylbutanal may also be demanded by the market.

The pentanal mixture obtained after the hydroformylation stage and aftercatalyst removal, which can also be regarded as crude hydroformylationproduct, is subsequently separated into a more volatile stream enrichedwith 2-methylbutanal and with 3-methylbutanal, and a less volatilestream enriched with n-pentanal, appropriately by distillation. Thedistillation of the crude hydroformylation product is effected byconventional methods in a distillation column. In order to obtainn-pentanal of maximum purity in the less volatile stream, the separationsharpness in the distillation is generally chosen such that the morevolatile stream enriched with 2-methylbutanal and with 3-methylbutanalobtained likewise still contains amounts of n-pentanal. The exactcomposition thereof depends on the composition of the butene feedmixture, the hydroformylation conditions and the distillationconditions; for example, the composition of this volatile stream may be80 to 85 mol % of 2-methyl-butanal, 10 to 14 mol % of n-pentanal and 1to 10 mol % of 3-methylbutanal, based on the pentanal content. Accordingto the isobutene content in the butene feed mixture, the 3-methylbutanalcontent may alternatively be higher or lower. In the less volatilestream, n-pentanal is concentrated in the virtual absence of the otherpentanal isomers.

While n-pentanal having a boiling point of 103° C. at standard pressurecan be separated from this volatile stream by a further conventionaldistillation in a further column having 10 to 100 trays as a product ofhigh purity, 2-methylbutanal having a boiling point of 92° C. atstandard pressure and 3-methylbutanal having a boiling point of 92.5° C.at standard pressure have too small a boiling point difference forsufficient separation with acceptable distillation complexity. Thisfurther distillation for n-pentanal removal can optionally be conducted.

According to the invention, the more volatile stream which is obtainedafter distillation of the crude hydroformylation product and has themain 2-methyl-butanal constituent and the residual 3-methylbutanal andn-pentanal contents is admixed with formaldehyde. The treatment ofα-alkyl-substituted aldehydes with formaldehyde to remove residualamounts of aldehydes having two hydrogen atoms on the α-carbon atomrelative to the carbonyl group in the course of purification is alsoreferred to as the methylenation reaction and is known per se from theprior art and is described, for example, in DE 3842186 A1 and DE 3744212A1.

The mixture comprising 2-methylbutanal, 3-methyl-butanal and n-pentanalis reacted with formaldehyde in the presence of an aldolizationcatalyst, typically a mixture of a secondary amine, for example an amineof the general formula R¹—NH—R² where R¹ and R² are the same ordifferent and are each alkyl radicals having 1 to 12 and preferably 3 to5 carbon atoms, and a monocarboxylic acid having 1 to 10 carbon atoms ora di- or polycarboxylic acid having 2 to 10 carbon atoms. Preference isgiven to using, as aldolization catalyst, a mixture of di-n-butylamineand n-butyric acid. However, other aldolization catalysts for themethylenation step are not ruled out. Formaldehyde is used in solidform, as paraformaldehyde, or appropriately as an aqueous solution incommercial concentration, such as 30% to 50% by weight, in which casethe molar ratio of formaldehyde to the sum total of aldehydes having twohydrogen atoms on the α-carbon atom relative to the carbonyl group is 1to 2.

The reaction is typically conducted at temperatures of 0 to 100° C.under autogenous pressure or slightly elevated pressure. Suitablereactors are the aggregates customary in chemical engineering, such asstirred tanks, stirred tank cascades, mixing pumps or flow tubes. Eithera batchwise or a continuous reaction regime is possible. Flow tubes areparticularly suitable for the continuous reaction regime, for example anupright or a horizontal flow tube or a multiply coiled flow tube. Theflow tube may be operated as an empty tube, but it may likewise containrandom packings or internals for intensive mixing of the organic phasewith the aqueous phase, for example Raschig rings, saddles, Pall rings,helices, baffles or static mixers or mixer packings. Static mixingelements are commercially available and are supplied, for example, inthe form of Sulzer mixers or Kenicks mixers. An appropriate spacevelocity in the flow tube of the mixture of organic phase comprising2-methylbutanal, 3-methylbutanal and n-pentanal and aqueous formaldehydesolution has been found to be from 0.1 to 10 h⁻¹, preferably from 0.1 to5 h⁻¹ and especially from 0.1 to 3 h⁻¹, based on reactor volume andtime.

The mixture leaving the reaction vessel is guided into a phase separatorin which the organic phase is separated from the aqueous phase.

In the methylenation, formaldehyde and the secondary amine in the acidicmedium at first form a Mannich salt which is preferably dehydrated with3-methylbutanal and n-pentanal via the formation of the α-methylolderivative to give isopropylacrolein and n-propylacrolein, while2-methylbutanal remains unchanged. Isopropylacrolein has a boiling pointof 108.5° C. at standard pressure and n-propylacrolein boils at 117° C.at standard pressure, and both methylenation products can be separatedfrom the 2-methylbutanal by distillation by conventional methods. The3-methylbutanal content in the 2-methylbutanal removed is generallybelow 0.2% by weight, based on the organic component. This is optionallyfollowed by another, fine distillation of the 2-methylbutanal.

The purified 2-methylbutanal obtained is subsequently oxidized to2-methylbutyric acid. The oxidation of 2-methylbutanal is known per se(Ullmann's Encyclopedia of Industrial Chemistry, 6th edition, 2003,Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, vol. 6, pages 497-502) andis preferably conducted in the liquid phase, for example in tubularreactors provided with a distributor tray, although other processconfigurations such as oxidation in the gas phase are not ruled out.Suitable oxidizing agents are customary compounds suitable for oxidationof aliphatic aldehydes, such as oxygen, oxygen-containing gas mixtures,ozone, ozone-containing gas mixtures, peroxides, peracids, metal saltsof peracids or transition metals in high oxidation states, for examplepotassium permanganate or manganese dioxide. Because of the goodavailability, oxidizing agents used are appropriately molecular oxygenor gas mixtures comprising molecular oxygen. Further constituents ofsuch gas mixtures are inert gases, for example nitrogen, noble gases andcarbon dioxide. The proportion of the inert constituents of theoxygen-containing gas mixture is up to 90% by volume, especially 30% to80% by volume. The preferred oxidizing agents are oxygen or air.

The oxidation can be conducted either with addition of catalysts or inthe absence of catalysts. Suitable catalysts are transition metals orcompounds of transition metals, which may be added in small amounts, forexample of 0.1 to 5 ppm, calculated as the transition metal and based onthe aldehyde used, such as titanium, vanadium, chromium, molybdenum,manganese, iron, cobalt, nickel, ruthenium, rhodium, palladium orcopper. Such a process regime is described, for example, in DE 100 10771 C1 or DE 26 04 545 A1.

It is likewise possible to conduct the conversion in the presence ofalkali metal or alkaline earth metal salts of weak acids. Especially inthe case of oxidation of α-branched aldehydes in which the carbon atomadjacent to the carbonyl carbon atom bears the branch, the prior artrecommends the presence of small amounts of alkali metal carboxylates toimprove selectivity (DE 950 007, DE 100 10 771 C1). Appropriately, theoxidation is conducted in the presence of 1 to 30 mmol, preferably 1 to15 mmol and especially 1 to 8 mmol per mole of aldehyde, calculated asalkali metal or alkaline earth metal. It is also possible to use acombination of alkali metal and alkaline earth metal carboxylates withtransition metal compounds, as discussed in EP 1 854 778 A1.

It is not necessary to use the alkali metal or alkaline earth metalcarboxylates in the form of a homogeneous compound. It is likewisepossible to use mixtures of these compounds, although it is appropriateto use 2-methylbutyrates. Preference is given, however, to usinghomogeneous compounds, for example lithium 2-methylbutyrate, potassium2-methylbutyrate, sodium 2-methylbutyrate, calcium 2-methylbutyrate orbarium 2-methylbutyrate.

In general, a solution comprising alkali metal or alkaline earth metal2-methylbutyrates is prepared by neutralizing an aqueous solutioncomprising alkali metal or alkaline earth metal compound with an excessof 2-methylbutyric acid and adding this solution to the 2-methylbutanalto be oxidized. Suitable alkali metal or alkaline earth metal compoundsare particularly the hydroxides, carbonates or hydrogencarbonates.

Alternatively, it is possible to produce the alkali metal or alkalineearth metal 2-methylbutyrates in the reaction mixture by adding alkalimetal or alkaline earth metal compounds which are converted under thereaction conditions to the 2-methylbutyrates. For example, it ispossible to use alkali metal or alkaline earth metal hydroxides,carbonates, hydrogencarbonates or oxides in the oxidation reaction. Theycan be added either in solid form or as an aqueous solution.

The reaction with the oxidizing agent, preferably with oxygen oroxygen-containing gases, is conducted within a temperature range of 20to 100° C. Preference is given to working between 20 and 80° C.,especially between 40 and 80° C. The temperature regime, constant orvariable temperature, can be matched to the individual requirements ofthe starting material and the circumstances of the reaction.

The conversion of the co-reactants is preferably effected at atmosphericpressure. However, the employment of elevated pressure is not ruled out.It is customary to work within a range from atmospheric pressure to 1.5MPa, preferably at atmospheric pressure to 0.8 MPa.

The oxidation step can be conducted batchwise or continuously. Recyclingof unconverted reaction participants is possible in both cases.

The acid obtained is subsequently distilled by distillation to giveon-spec material. The purity of the 2-methylbutyric acid is generallyabove 99.7% by weight and the residual content of 3-methylbutyric acidis less than 0.2% by weight, based on the organic component.2-Methylbutyric acid of this quality is outstandingly suitable as anacid component for the preparation of ester lubricants and fragrances.

In the process of the invention, after the reaction of the2-methylbutanal- and 3-methyl-butanal-enriched volatile stream whichlikewise still contains residual amounts of n-pentanal with formaldehydein the presence of the aldolization catalyst in step c), the organicphase is separated from the aqueous phase and the crude organic phase isselectively hydrogenated in the presence of a hydrogenation catalystwith hydrogen at elevated temperature and elevated pressure. Thissaturates the double bond in the α, β position relative to the carbonylcarbon atom with retention of the aldehyde group, such that the outcomeof the selective hydrogenation is that a mixture comprising unchanged2-methylbutanal, 2-methylpentanal from the selective hydrogenation ofn-propylacrolein and 2,3-dimethylbutanal from the selectivehydrogenation of isopropylacrolein is obtained. The selectivehydrogenation is conducted in a known manner over supported orunsupported catalysts containing, as hydrogenation-active component,palladium, platinum, rhodium and/or nickel. Preference is given toworking with palladium catalysts at temperatures of 120 to 180° C.,preferably 140 to 160° C., and at a pressure of 1.5 to 5 MPa, preferablyat 2 to 3 MPa.

The hydrogenation output obtained is optionally separated from thehydrogenation catalyst and then, optionally after removal of low andhigh boilers, oxidized. The oxidation conditions corresponding to theaforementioned conditions. What is obtained is a crude acid mixturewhich, as well as 2-methylbutyric acid, boiling point 177° C., alsocontains 2-methylpentanoic acid, boiling point 196° C., and2,3-dimethylbutyric acid, boiling point 191° C., each at standardpressure. After distillation under conventional conditions,2-methylbutyric acid is obtained with a purity of more than 99.7% byweight and with a residual 3-methylbutyric acid content of less than0.2% by weight, based on the organic component.

In a configuration of the process of the invention, after thehydrogenation catalyst has been removed, the output from the selectivehydrogenation is fractionally distilled. 2-Methylbutanal is separated asa more volatile component from the 2-methylpentanal and any2,3-dimethylbutanal present. The aldehyde fractions separated weresubsequently treated with an oxidizing agent under the conditionsdescribed above. After purification by distillation, 2-methylbutyricacid is obtained with a residual 3-methylbutyric acid content of lessthan 0.2% by weight, based on the organic component. 2-Methylpentanoicacid obtained as a separate fraction, which may likewise still contain2,3-dimethylbutyric acid.

The process of the invention enables the preparation of 2-methylbutyricacid in high quality from secondary streams obtained in the preparationof pentanals. Because of the low residual 3-methylbutyric acid content,the 2-methylbutyric acid obtained is outstandingly suitable as an acidcomponent in ester compounds. Ester compounds with lower monoalcoholsare used for the production of fragrances, while ester compounds ofpolyols are used for the production of synthetic lubricants.

The high-purity 2-methylbutyric acid having a 3-methylbutyric acidcontent of less than 0.2% by weight can likewise be processed togetherwith high-purity n-pentanoic acid to give a high-purity acid mixturewhich can also be referred to as isopentanoic acid.

The invention claimed is:
 1. A process for preparing 2-methylbutyricacid having a reduced content of 3-methylbutyric acid from secondarystreams obtained in the preparation of pentanoic acids, characterized inthat a) a mixture comprising linear butenes is reacted in the presenceof transition metal compounds of group VIII of the Periodic Table of theElements with carbon monoxide and hydrogen at elevated temperature andelevated pressure to give a pentanal mixture; b) the mixture obtained instep a) is separated into a stream enriched with 2-methylbutanal and3-methyl-butanal, and a stream enriched with n-pentanal; c) the streamenriched with 2-methylbutanal and 3-methylbutanal of step b) is reactedwith formaldehyde; d) the reaction product obtained after step c) istreated with an oxidizing agent; and e) 2-methylbutyric acid having aresidual 3-methylbutyric acid content of less than 0.2% by weight, basedon the organic component, is removed by distillation from the reactionproduct obtained in step d); with the proviso that the reaction withformaldehyde in step c) is followed by removal of the organic phase andselective hydrogenation in the presence of a hydrogenation catalyst withhydrogen at elevated temperature and elevated pressure and, afterremoval of the hydrogenation catalyst, treatment of the hydrogenationoutput obtained with an oxidizing agent in step d).
 2. The process asclaimed in claim 1, characterized in that the reaction with formaldehydein step c) is effected in the presence of a secondary amine and a mono-,di- or polycarboxylic acid.
 3. The process as claimed in claim 1,characterized in that, after removal of the hydrogenation catalyst, thehydrogenation output obtained is fractionally distilled and theseparated fractions are treated with an oxidizing agent in step d). 4.The process as claimed in claim 1, characterized in that the secondaryamine used is an alkylamine of the formula R¹—NH—R² where R¹ and R² arethe same or different and are each alkyl radicals having 1 to 12 carbonatoms.
 5. The process as claimed in claim 1, characterized in that amonocarboxylic acid having 1 to 10 carbon atoms or a di- orpolycarboxylic acid having 2 to 10 carbon atoms is used.
 6. The processas claimed in claim 1, characterized in that the oxidation in step d) iseffected in the presence of alkali metal or alkaline earth metalcarboxylates.
 7. The process as claimed in claim 6, characterized inthat the alkali metal or alkaline earth metal carboxylates used arelithium carboxylate, potassium carboxylate, sodium carboxylate, calciumcarboxylate or barium carboxylate.
 8. The process as claimed in claim 1,characterized in that oxidation is effected in the liquid phase in stepd).
 9. The process as claimed in claim 1, characterized in that theoxidizing agent used in step d) is oxygen or oxygen-containing gases.10. The process as claimed in claim 1, characterized in that thereaction with formaldehyde in step c) is conducted continuously in aflow tube.
 11. The process as claimed in claim 2, characterized in thatthe secondary amine used is an alkylamine of the formula R¹—NH—R² whereR¹ and R² are the same or different and are each alkyl radicals having 1to 12 carbon atoms.
 12. The process as claimed in claim 3, characterizedin that the secondary amine used is an alkylamine of the formulaR¹—NH—R² where R¹ and R² are the same or different and are each alkylradicals having 1 to 12 carbon atoms.
 13. The process as claimed inclaim 1, characterized in that the secondary amine used is an alkylamineof the formula R¹—NH—R² where R¹ and R² are the same or different andare each alkyl radicals having 3 to 5 carbon atoms.
 14. The process asclaimed in claim 2, characterized in that a monocarboxylic acid having 1to 10 carbon atoms or a di- or polycarboxylic acid having 2 to 10 carbonatoms is used.
 15. The process as claimed in claim 3, characterized inthat a monocarboxylic acid having 1 to 10 carbon atoms or a di- orpolycarboxylic acid having 2 to 10 carbon atoms is used.
 16. The processas claimed in claim 4, characterized in that a monocarboxylic acidhaving 1 to 10 carbon atoms or a di- or polycarboxylic acid having 2 to10 carbon atoms is used.
 17. The process as claimed in claim 2,characterized in that the oxidation in step d) is effected in thepresence of alkali metal or alkaline earth metal carboxylates.
 18. Theprocess as claimed in claim 3, characterized in that the oxidation instep d) is effected in the presence of alkali metal or alkaline earthmetal carboxylates.
 19. The process as claimed in claim 4, characterizedin that the oxidation in step d) is effected in the presence of alkalimetal or alkaline earth metal carboxylates.
 20. The process as claimedin claim 5, characterized in that the oxidation in step d) is effectedin the presence of alkali metal or alkaline earth metal carboxylates.